The replacement of incandescent bulbs by light emitting diode (LED) lighting devices reduces energy consumption due to the high efficiency of LED devices as compared to incandescent bulbs. But an incandescent bulb may be directly driven by the AC mains whereas an LED device cannot. It is thus conventional for an LED device to include a switching power converter such as a flyback converter to provide a regulated DC current for driving the LED. This need for a switching power converter raises costs and thus diminishes a consumer's desire to switch to LED devices and instead continue to use incandescent bulbs, which contributes to global warming due to the increased greenhouse gas emissions from the resulting energy consumption.
To lower LED device costs, direct AC LED devices (which may also be denoted as direct AC LED bulbs) have been developed that obviate the need for a switching power converter. In a direct AC LED device, the AC mains voltage is rectified through a rectifier such as a bridge diode rectifier to produce a rectified AC input voltage. The LED in a direct AC LED device is directly driven by the rectified AC input voltage. Although no switching power converter is thus needed to convert the rectified AC input voltage into a regulated DC current/voltage, a direct AC LED device still needs a controller to control the LED power. In particular, the controller controls a current source in series with the LED. When the rectified AC input voltage (which may also be denoted as a post diode bridge voltage) rises above the threshold voltage for the LED, the controller switches on the current source so that the LED conducts a substantially constant current set by the current source. The resulting post diode bridge voltage and LED current waveforms are shown in FIG. 1. The post diode bridge voltage cycles high two times for every cycle of the AC mains voltage (not illustrated). When the post diode bridge voltage exceeds the LED threshold voltage, the LED current flows until the post diode bridge voltage falls below the LED threshold voltage again. The resulting pulsing of the LED current causes substantial total harmonic distortion (THD) and lowered power factor.
Accordingly, there is a need in the art for direct AC LED devices with improved THD and improved power factor.